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          <h1 class="post-title" itemprop="name headline">Java的对象模型</h1>
        

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        <p>众所周知，在JVM的内存结构中，对象保存在堆内存中，而我们在对对象进行操作时，其实操作的是对象的引用。那么对象本身在JVM中的结构是什么样的呢？</p>
<p>HotSpot是基于c++实现，而c++是一门面向对象的语言，HotSpot JVM为了实现面向对象的设计，设计了一个<code>OOP-Klass Model</code>。OOP（<code>Ordinary Object Pointer</code>）指的是普通对象指针，而<code>Klass</code>是用来描述对象实例的具体类型。</p>
<h3 id="OOP的定义"><a href="#OOP的定义" class="headerlink" title="OOP的定义"></a>OOP的定义</h3><p>在JVM中，对象在内存中的基本存在形式就是oop，<strong>在Java程序运行过程中，每创建一个新的对象，在JVM内部就会相应地创建一个对应类型的OOP对象。</strong><br>这些OOP在JVM内部有着不同的用途，例如，<code>instanceOopDesc</code>表示类实例，<code>arrayOopDesc</code>表示数组。也就是说，当我们使用<code>new</code>创建一个Java对象实例的时候，JVM会创建一个<code>instanceOopDesc</code>对象来表示这个Java对象。同理，当我们使用<code>new</code>创建一个Java数组实例的时候，JVM会创建一个<code>arrayOopDesc</code>对象来表示这个数组对象。</p>
<p>整个OOP模块的组成包含很多个子模块，每一个子模块对应一个类型，每一个类型的OOP都代表一个在JVM内部使用的特定对象的类型。<br>oop体系：<br><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//定义了oop的共同基类</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">oopDesc</span>*	<span class="title">op</span>;</span></span><br><span class="line"><span class="comment">//表示一个Java类型实例</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">instanceOopDesc</span>*	<span class="title">instanceOop</span>;</span></span><br><span class="line"><span class="comment">//表示一个Java方法</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">methodOopDesc</span>*	<span class="title">thodOop</span>;</span></span><br><span class="line"><span class="comment">//表示一个Java方法中的不变信息</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">constMethodOopDesc</span>*	<span class="title">stMethodOop</span>;</span></span><br><span class="line"><span class="comment">//记录性能信息的数据结构</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">methodDataOopDesc</span>*	<span class="title">thodDataOop</span>;</span></span><br><span class="line"><span class="comment">//定义了数组的抽象基类</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">arrayOopDesc</span>*	<span class="title">rrayOop</span>;</span></span><br><span class="line"><span class="comment">//表示持有一个对象数组</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">objArrayOopDesc</span>*	<span class="title">objArrayOop</span>;</span></span><br><span class="line"><span class="comment">//表示容纳基本类型的数组</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">typeArrayOopDesc</span>*	<span class="title">ypeArrayOop</span>;</span></span><br><span class="line"><span class="comment">//表示在Class文件中描述的常量池</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">constantPoolOopDesc</span>*	<span class="title">constantPoolOop</span>;</span></span><br><span class="line"><span class="comment">//常量池高速缓存</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">constantPoolCacheOopDesc</span>*	<span class="title">onstantPoolCacheOop</span>;</span></span><br><span class="line"><span class="comment">//描述一个与Java类对等的C++类</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">klassOopDesc</span>*	<span class="title">lassOop</span>;</span></span><br><span class="line"><span class="comment">//表示对象头</span></span><br><span class="line"><span class="keyword">typedef</span>	<span class="class"><span class="keyword">class</span>	<span class="title">markOopDesc</span>*	<span class="title">markOop</span>;</span></span><br></pre></td></tr></table></figure></p>
<p>从上面的代码中可以看到，有一个变量opp的类型是oppDesc ，OOPS类的共同基类型为oopDesc。<br>看一下oppDesc的相关定义：<br><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">oopDesc</span> &#123;</span></span><br><span class="line">  <span class="keyword">friend</span> <span class="class"><span class="keyword">class</span> <span class="title">VMStructs</span>;</span></span><br><span class="line">  <span class="keyword">private</span>:</span><br><span class="line">      <span class="keyword">volatile</span> markOop  _mark;</span><br><span class="line">      <span class="keyword">union</span> _metadata &#123;</span><br><span class="line">        wideKlassOop    _klass;</span><br><span class="line">        narrowOop       _compressed_klass;</span><br><span class="line">      &#125; _metadata;</span><br><span class="line"></span><br><span class="line">  <span class="keyword">private</span>:</span><br><span class="line">      <span class="comment">// field addresses in oop</span></span><br><span class="line">      <span class="function"><span class="keyword">void</span>*     <span class="title">field_base</span><span class="params">(<span class="keyword">int</span> offset)</span>        <span class="keyword">const</span></span>;</span><br><span class="line"></span><br><span class="line">      <span class="function">jbyte*    <span class="title">byte_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>   <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jchar*    <span class="title">char_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>   <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jboolean* <span class="title">bool_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>   <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jint*     <span class="title">int_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>    <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jshort*   <span class="title">short_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>  <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jlong*    <span class="title">long_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>   <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jfloat*   <span class="title">float_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span>  <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">jdouble*  <span class="title">double_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span> <span class="keyword">const</span></span>;</span><br><span class="line">      <span class="function">address*  <span class="title">address_field_addr</span><span class="params">(<span class="keyword">int</span> offset)</span> <span class="keyword">const</span></span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">instanceOopDesc</span> :</span> <span class="keyword">public</span> oopDesc &#123;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">arrayOopDesc</span> :</span> <span class="keyword">public</span> oopDesc &#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>可以看到，OOP主要包含以下几部分数据：<code>markOop _mark</code>和<code>union _metadata</code>以及一些不同类型的<code>field</code>。</p>
<p>在虚拟机内部，一个Java对象对应一个<code>instanceOopDesc</code>的对象，对象在内存中存储的布局可以分为三块区域：对象头、实例数据和对齐填充。</p>
<ul>
<li>对象头：·存放对象头部信息，采用2个字来存储对象头（如果对象是数组则会分配3个字，多出来的1个字记录的是数组长度）。</li>
<li>实例数据：存放类的属性数据信息，包括父类的属性信息，如果是数组的实例部分还包括数组的长度，这部分内存按4字节对齐，保存在oopDesc中定义的各种field中。</li>
<li>填充数据：由于虚拟机要求对象起始地址必须是8字节的整数倍。填充数据不是必须存在的，仅仅是为了字节对齐。</li>
</ul>
<p>对象头主要是由<code>_mark</code>（Mark Word）和<code>_metadata</code>（Class Metadata Address）组成，其结构如下：<br>
<table>
    <thead>
    <tr>
        <th>头对象结构</th>
        <th>说明</th>
    </tr>
    </thead>
    <tbody>
    <tr>
        <td>Mark Word</td>
        <td>存储对象的hashCode、锁信息、分代年龄、GC标志等信息</td>
    </tr>
    <tr>
        <td>Class Metadata Address</td>
        <td>类型指针，指向对象的类元数据，JVM通过这个指针确定该对象是哪个类的实例。</td>
    </tr>
    </tbody>
</table>
</p>
<h4 id="Mark-Word"><a href="#Mark-Word" class="headerlink" title="Mark Word"></a>Mark Word</h4><p>Mark Word在默认情况下存储着对象的HashCode、分代年龄、锁标记位等信息。<br>由于对象头的信息是与对象自身定义的数据没有关系的额外存储成本，因此考虑到JVM的空间效率，Mark Word 被设计成为一个非固定的数据结构，以便存储更多有效的数据，它会根据对象本身的状态复用自己的存储空间。</p>
<p>对Mark Word的设计方式上，非常像网络协议报文头：将Mark Word划分为多个比特位区间，并在不同的对象状态下赋予比特位不同的含义。比如32位JVM下，Mark Word的结构：<br>
<table>
    <thead>
    <tr>
        <th rowspan="2">锁状态</th>
        <th colspan="2">25 bit</th>
        <th rowspan="2">4 bit</th>
        <th>1 bit</th>
        <th>2 bit</th>
    </tr>
    <tr>
        <th>23 bit</th>
        <th>2 bit</th>
        <th>是否偏向锁</th>
        <th>锁标志位</th>
    </tr>
    </thead>
    <tbody>
    <tr>
        <td>无锁态</td>
        <td colspan="2">对象的hashCode</td>
        <td>分代年龄</td>
        <td>0</td>
        <td>01</td>
    </tr>
    <tr>
        <td>轻量级锁</td>
        <td colspan="4">指向栈中锁记录的指针</td>
        <td>00</td>
    </tr>
    <tr>
        <td>重量级锁</td>
        <td colspan="4">指向互斥锁（重量级锁）的指针</td>
        <td>10</td>
    </tr>
    <tr>
        <td>GC标记</td>
        <td colspan="4">空</td>
        <td>11</td>
    </tr>
    <tr>
        <td>偏向锁</td>
        <td>线程ID</td>
        <td>Epoch</td>
        <td>分代年龄</td>
        <td>1</td>
        <td>01</td>
    </tr>
    </tbody>
</table>
</p>
<p>从上图中可以看出，对象的状态一共有五种，分别是无锁态、轻量级锁、重量级锁、GC标记和偏向锁。重量级锁也就是通常说<code>synchronized</code>的对象锁，锁标识位为10，其中指针指向的是 Monitor 对象（也称为管程或监视器锁）的起始地址。</p>
<p>关于监视器锁的实现原理，在我另一篇文章中有简单介绍：<a href="/blog/Multithreading_monitor.html" title="监视器锁的实现原理">监视器锁的实现原理</a>。</p>
<h3 id="Class-Metadata-Address"><a href="#Class-Metadata-Address" class="headerlink" title="Class Metadata Address"></a>Class Metadata Address</h3><p>在oopDesc中<code>_metadata</code>定义如下：<br><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">oopDesc</span> &#123;</span></span><br><span class="line">  <span class="keyword">friend</span> <span class="class"><span class="keyword">class</span> <span class="title">VMStructs</span>;</span></span><br><span class="line">  <span class="keyword">private</span>:</span><br><span class="line">      <span class="keyword">volatile</span> markOop  _mark;</span><br><span class="line">      <span class="keyword">union</span> _metadata &#123;</span><br><span class="line">        wideKlassOop    _klass;</span><br><span class="line">        narrowOop       _compressed_klass;</span><br><span class="line">      &#125; _metadata;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>其中<code>_klass</code>是普通指针，<code>_compressed_klass</code>是压缩类指针，这两个指针都指向<code>instanceKlass</code>对象，它用来描述对象的具体类型。。</p>
<p>JVM在运行时，需要一种用来标识Java内部类型的机制。在HotSpot中的解决方案是：为每一个已加载的Java类创建一个<code>instanceKlass</code>对象，用来在JVM层表示Java类。就像<code>instanceOopDesc</code>一样，<code>instanceKlass</code>是众多<code>Klass</code>中的一员。<br>klass体系：<br><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//klassOop的一部分，用来描述语言层的类型</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">Klass</span>;</span></span><br><span class="line"><span class="comment">//在虚拟机层面描述一个Java类</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">instanceKlass</span>;</span></span><br><span class="line"><span class="comment">//专有instantKlass，表示java.lang.Class的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">instanceMirrorKlass</span>;</span></span><br><span class="line"><span class="comment">//专有instantKlass，表示java.lang.ref.Reference的子类的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">instanceRefKlass</span>;</span></span><br><span class="line"><span class="comment">//表示methodOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">methodKlass</span>;</span></span><br><span class="line"><span class="comment">//表示constMethodOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">constMethodKlass</span>;</span></span><br><span class="line"><span class="comment">//表示methodDataOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">methodDataKlass</span>;</span></span><br><span class="line"><span class="comment">//最为klass链的端点，klassKlass的Klass就是它自身</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">klassKlass</span>;</span></span><br><span class="line"><span class="comment">//表示instanceKlass的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">instanceKlassKlass</span>;</span></span><br><span class="line"><span class="comment">//表示arrayKlass的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">arrayKlassKlass</span>;</span></span><br><span class="line"><span class="comment">//表示objArrayKlass的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">objArrayKlassKlass</span>;</span></span><br><span class="line"><span class="comment">//表示typeArrayKlass的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">typeArrayKlassKlass</span>;</span></span><br><span class="line"><span class="comment">//表示array类型的抽象基类</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">arrayKlass</span>;</span></span><br><span class="line"><span class="comment">//表示objArrayOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">objArrayKlass</span>;</span></span><br><span class="line"><span class="comment">//表示typeArrayOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">typeArrayKlass</span>;</span></span><br><span class="line"><span class="comment">//表示constantPoolOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">constantPoolKlass</span>;</span></span><br><span class="line"><span class="comment">//表示constantPoolCacheOop的Klass</span></span><br><span class="line"><span class="class"><span class="keyword">class</span>	<span class="title">constantPoolCacheKlass</span>;</span></span><br></pre></td></tr></table></figure></p>
<p>instanceKlass的内部结构：<br><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//类拥有的方法列表</span></span><br><span class="line">objArrayOop     _methods;</span><br><span class="line"><span class="comment">//描述方法顺序</span></span><br><span class="line">typeArrayOop    _method_ordering;</span><br><span class="line"><span class="comment">//实现的接口</span></span><br><span class="line">objArrayOop     _local_interfaces;</span><br><span class="line"><span class="comment">//继承的接口</span></span><br><span class="line">objArrayOop     _transitive_interfaces;</span><br><span class="line"><span class="comment">//域</span></span><br><span class="line">typeArrayOop    _fields;</span><br><span class="line"><span class="comment">//常量</span></span><br><span class="line">constantPoolOop _constants;</span><br><span class="line"><span class="comment">//类加载器</span></span><br><span class="line">oop             _class_loader;</span><br><span class="line"><span class="comment">//protected域</span></span><br><span class="line">oop             _protection_domain;</span><br><span class="line">      ....</span><br></pre></td></tr></table></figure></p>
<p>可以看到，一个类该具有的东西，这里面基本都包含了。Klass向JVM提供两个功能：</p>
<ul>
<li>实现语言层面的Java类（在Klass基类中已经实现）</li>
<li>实现Java对象的分发功能（由Klass的子类提供虚函数实现）</li>
</ul>
<p>在JVM中，对象在内存中的基本存在形式就是oop。那么，对象所属的类，在JVM中也是一种对象，因此它们实际上也会被组织成一种oop，即klassOop。同样的，对于klassOop，也有对应的一个klass来描述，它就是klassKlass，也是klass的一个子类。klassKlass作为oop的klass链的端点。关于对象和数组的klass链大致如下图：<br><img src="/blog/JVM_object_model/1.png"><br>在这种设计下，JVM对内存的分配和回收，都可以采用统一的方式来管理。oop-klass-klassKlass关系如图：<br><img src="/blog/JVM_object_model/2.png"></p>
<p>我们都知道，方法区用于存储虚拟机加载的类信息、常量、静态变量、即时编译器编译后的代码等数据。 所谓加载的类信息，其实就是给每一个被加载的类都创建了一个<code>instantKlass</code>对象。所以对象在JVM中<strong>对象的实例（instantOopDesc)保存在堆上，对象的元数据（instantKlass）保存在方法区，对象的引用保存在栈上。</strong></p>
<p>举个例子：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Model</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">int</span> a = <span class="number">1</span>;</span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">int</span> b;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Model</span><span class="params">(<span class="keyword">int</span> b)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.b = b;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">    <span class="keyword">int</span> c = <span class="number">10</span>;</span><br><span class="line">    Model modelA = <span class="keyword">new</span> Model(<span class="number">2</span>);</span><br><span class="line">    Model modelB = <span class="keyword">new</span> Model(<span class="number">3</span>);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>存储结构如下：<br><img src="/blog/JVM_object_model/3.png"><br>从上图中可以看到，在方法区的instantKlass中有一个int a=1的数据存储。在堆内存中的两个对象的oop中，分别维护着int b=3,int b=2的实例数据。和oopDesc一样，instantKlass也维护着一些fields，用来保存类中定义的类数据，比如int a=1。</p>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>每一个Java类，在被JVM加载的时候，JVM会给这个类创建一个instanceKlass，保存在方法区，用来在JVM层表示该Java类。<br>当我们在Java代码中，使用new创建一个对象的时候，JVM会创建一个instanceOopDesc对象，这个对象中包含了两部分信息，对象头以及元数据。对象头中有一些运行时数据，其中就包括和多线程相关的锁的信息。元数据其实维护的是指针，指向的是对象所属的类的instanceKlass。</p>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#OOP的定义"><span class="nav-number">1.</span> <span class="nav-text">OOP的定义</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#Mark-Word"><span class="nav-number">1.1.</span> <span class="nav-text">Mark Word</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#Class-Metadata-Address"><span class="nav-number">2.</span> <span class="nav-text">Class Metadata Address</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#总结"><span class="nav-number">3.</span> <span class="nav-text">总结</span></a></li></ol></div>
            

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    // monitor main search box;

    var onPopupClose = function (e) {
      $('.popup').hide();
      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
      $(".local-search-pop-overlay").remove();
      $('body').css('overflow', '');
    }

    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
    }

    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
          '<i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>' +
          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  

  

  

  

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